The structural influence on translational diffusion of 2,2,4-trimethylpentane (TMP) through SEBS triblock polymer (poly(styrene)-block-poly(ethene-co-but-1-ene)-block-poly(styrene)) was studied using pulse field gradient (PFG) NMR coupled with lattice model simulation. Two types of PFG experiments were performed:  one observing the time-dependent apparent diffusion constant and another observing the diffusion-induced NMR signal attenuation. TMP is selectively sorbed into the rubbery ethylene/butylenes (EB) phase while the glassy poly(styrene) (PS) phase acts as a barrier to TMP diffusion. The observed apparent diffusion constant drops drastically at the grain boundary, in which the orientation of the minor EB cylinder phase changes from grain to grain. In the lattice model simulation, in order to match the large drop of diffusion constant, the diameter of the EB cylinder at the interface had to be reduced by a factor of about 0.7. This extra restriction in the size of the conductive phase indicates the important role of the grain boundary on diffusion in membrane applications of block copolymers. The simulation also shows that the grain boundary influence on diffusion becomes significant when the solubility and diffusivity of the penetrant are greatly different between the rubber and glass phases. In addition, we extended the lattice model to simulate the diffusion-induced PFG NMR signal attenuation. From the simulation and theoretical fitting, it is obvious that the EB phase at grain boundary is not connected well, which is in agreement with our observation of a drastic apparent diffusion constant drop at the grain boundaries in the PFG experiment.

Pauling and Corey proposed a pleated-sheet configuration, now called α-sheet, as one of the protein secondary structures in addition to α-helix and β-sheet. Recently, it has been suggested that α-sheet is a common feature of amyloidogenic intermediates. We have investigated the stability of anti-parallel β-sheet and two conformations of α-sheet in solution phase using the density functional theoretical method. The peptides are modeled as two-strand Acetyl-(Ala)2-N-methylamine. Using stages of geometry optimization and single point energy calculation at B3LYP/cc-pVTZ//B3LYP/6-31G* level and including zero-point energies, thermal, and entropic contribution, we have found that β-sheet is the most stable conformation, while the α-sheet proposed by Pauling and Corey has 13.6 kcal/mol higher free energy than the β-sheet. The α-sheet that resembles the structure observed in molecular dynamics simulations of amyloidogenic proteins at low pH becomes distorted after stages of geometry optimization in solution. Whether the α-sheets with longer chains would be increasingly favorable in water relative to the increase in internal energy of the chain needs further investigation. Different from the quantum mechanics results, AMBER parm94 force field gives small difference in solution phase energy between α-sheet and β-sheet. The predicted amide I IR spectra of α-sheet shows the main band at higher frequency than β-sheet.